In recent years, a self-light-emissive display device in which organic electroluminescence (hereinafter, referred to as “organic EL”) is used has been developed as a display device that can substitute for a liquid crystal display device.
An organic EL element can emit light with a voltage of approximately several volts to several tens of volts. The organic EL element is self-light-emissive and is therefore wide in viewing angle and high in viewability. Further, the organic EL element is a perfect solid element in a form of thin film, and therefore attracts attentions from the viewpoint of space saving, portability, and the like.
The organic EL element has a configuration in which a luminescent layer containing a luminescent material made of an organic compound is sandwiched between a cathode and an anode. The organic EL element emits light by utilizing a mechanism in which electrons and positive holes (holes) are introduced into the luminescent layer, the electrons and positive holes are caused to recombine so that excitons are generated thereby, and light is emitted when the excitons lose their activity.
The luminescent material is excited when organic molecules in a ground state (S0) absorb light energy and molecules at a highest occupied molecular orbital (HOMO) level are shifted to a lowest unoccupied molecular orbital (LUMO) level.
The organic molecule has two excitation states which are different in spin multiplicity, i.e., a singlet excitation state (S1) in which spin directions of the HOMO and the LUMO are parallel, and a triplet excitation state (T1) in which spin directions of the HOMO and the LUMO are antiparallel.
(a) of FIG. 11 is an explanatory view illustrating an exciton generation state of an ordinary luminescent material, and (b) of FIG. 11 is an explanatory view schematically illustrating an exciton generation state of a thermally activated delayed luminescent (thermally activated delayed fluorescent (TADF)) material which will be described later.
In general, the luminescent layer is constituted by a two-component system including (i) a host material which contributes to transfer of positive holes and electrons and (ii) a light emission dopant (guest) material which contributes to light emission. The light emission dopant is uniformly dispersed in the host material which is a main component.
As illustrated in (a) of FIG. 11, in an ordinary process of generating excitons in an organic EL element in which a luminescent material is used as a dopant, a singlet exciton which is an exciton in a singlet excitation state is generated at a probability of only 25%. At the remaining probability of 75%, a triplet exciton which is an exciton in a triplet excitation state is generated.
However, transition from the singlet excitation state to the ground state is transition between states having identical spin multiplicities, whereas transition from the triplet excitation state to the ground state is transition between states having different spin multiplicities.
Therefore, the transition from the triplet excitation state to the ground state is forbidden transition which requires a long time. As a result, the triplet exciton does not lose its activity for light emission but loses its activity for heat by changing into heat energy or the like, and thus does not contribute to light emission.
From this, although a conventional fluorescent emission material (hereinafter, sometimes simply referred to as “fluorescent material”) has many advantages such as an excellent high electric current density characteristic and variety in material selection, only 25% of the singlet exciton can be used in light emission.
Under the circumstances, in recent years, a thermally activated delayed luminescent (TADF) material has been developed in which a difference between energy in the singlet excitation state (excited singlet level: hereinafter referred to as “S1 level”) and energy in the triplet excitation state (excited triplet level: hereinafter referred to as “T1 level”) is extremely small. As illustrated in (b) of FIG. 11, studies are carried on for causing the triplet exciton to contribute to light emission by restoring the triplet exciton into the singlet exciton.
The development of the TADF material is carried on from both viewpoints of a TADF material used as a host material (TADF host material) and a TADF material used as a dopant (TADF dopant material). However transition (energy transfer) from an S1 level of the TADF host to an S1 level of the TADF dopant does not necessarily occur. For causing such transition, an energy relation between the S1 level of the TADF host material and the S1 level of the TADF dopant material is important. However, it is difficult to find a combination of TADF materials which have good compatibility.
On the other hand, there are many kinds of non-TADF materials. Therefore, the development can be made easy by utilizing a combination of a TADF material and a non-TADF material such as a combination of a TADF host material and an ordinary fluorescent material, and a combination of an ordinary host material (non-TADF host material) and a TADF dopant.
From this, so far, an organic EL element in which a TADF material and a non-TADF material are combined has been mainly developed. Each of Patent Literatures 1 and 2 discloses an organic EL element in which a luminescent layer contains a TADF material and a non-TADF material. The following description will discuss Patent Literature 1 as an example.
The organic EL element disclosed in Patent Literature 1 at least includes a luminescent layer provided between a first electrode and a second electrode. The luminescent layer at least contains a fluorescent material which is a luminescent substance and a TADF material.
From a singlet excitation state of the TADF material, energy transfer to a singlet excitation state of the fluorescent material occurs. Moreover, energy transfer from a triplet excitation state of the TADF material to a singlet excitation state to the fluorescent material A occurs via inverse intersystem crossing to the singlet excitation state of the TADF material. From this, Patent Literature 1 describes that light emission efficiently occurs from the singlet excitation state of the fluorescent material.